The present invention relates to an image forming apparatus of the type scooping up a developer to a developer carrier, causing the developer to form a magnet brush on the developer carrier, and causing the magnet brush to contact an image carrier to thereby develop a latent image formed on the image carrier. More particularly, the present invention relates to an image forming apparatus capable of preventing the developer from adhering to the developer carrier.
It is a common practice with a copier, printer, facsimile apparatus or similar electrophotographic or electrostatic recording type of image forming apparatus to use an image carrier implemented by, e.g., a photoconductive drum or a photoconductive belt. A latent image is electrostatically formed on the image carrier in accordance with image data. The latent image is developed by a developer to become a toner image.
Today, a magnet brush type developing system, which uses a toner and carrier mixture as a developer, is predominant over the other developing systems in the aspect of image transfer, halftone reproducibility, and stability against varying temperature and humidity. In this type of developing system, the toner and carrier mixture, i.e., two-ingredient type developer rises from a developer carrier in the form of brush chains. In a developing region, the toner contained in the developer is fed to a latent image formed on an image carrier to thereby form a corresponding toner image. The developing region refers to a range over which the magnet brush rises from the developer carrier and contacts the image carrier.
Usually, the developer carrier is implemented by a cylindrical sleeve. A magnet roller is disposed in the sleeve in order to cause the developer deposited on the sleeve to rise. More specifically, the carrier of the developer rises from the sleeve in the form of chains along the magnetic lines of force formed by the magnet roller. The carrier, which has been charged, deposits on the carrier rising from the sleeve. The magnet roller has a plurality of magnetic poles each being implemented by, e.g., a rod-like magnet. Particularly, a main magnetic pole for development is positioned in the developing region in order to cause the developer to rise. Either one of the sleeve and magnet roller is movable relative to the other so as to move the developer rising from the sleeve. In the developing region, the developer rises in the form of chains along the magnetic lines of force formed by the main pole and contacts the surface of the image carrier. The chains rub themselves against the latent image on the basis of a difference in linear velocity between the chains and the image carrier, thereby feeding the toner to the latent image.
The distance between the developer carrier and the image carrier at a point where they are closest to each other, i.e., a gap for development has critical influence on image quality. If the gap is excessively great, then the amount of toner to deposit on the edges of the latent image increases, resulting in an undesirable edge-enhanced image. While this may be coped with by reducing the gap, an excessively small gap extends an electric field for development and thereby increases development gamma. Development gamma should be confined in an adequate range because it would deteriorate tonality if excessively high of would obstruct the deposition of the maximum amount of toner is excessively low.
Development gamma is proportional to the linear velocity ratio of the sleeve, i.e., the ratio of the peripheral speed of the sleeve to that of the image carrier. To lower excessively high development gamma to a desired value, there may be reduced the rotation speed of the sleeve and therefore the linear velocity ratio of the sleeve. Although this kind of scheme recovers tonality, it lowers the developing ability. Consequently, at a nip where the magnet brush on the sleeve rubs itself against the image carrier, the pressure of the developer acting on the image carrier increases and causes the toner to adhere.
The adhered toner causes the torque for rotating the developer carrier to increase, so that irregular drive and therefore banding occurs at the gear pitch of a driveline including a drive gear. At the same time, the adhered toner makes the drive of the image carrier irregular. In the case of a latent image formed by, e.g., a laser in the form of dots, the irregular drive of the image carrier results in irregular distance between the dots and therefore banding. Moreover, if image formation is continued in the above conditions, the toner is apt to shave off the photoconductive layer of the image carrier or to stop the drive of the image carrier due to the difference in linear velocity between the image carrier and the sleeve.
Another conventional scheme for the reduction of the edge-enhanced image is to use an AC bias as a bias for development. The AC bias lowers the electric resistance of the chains of the magnet brush and thereby substantially reduce the gap for development. The AC bias, however, increases development gamma. Again, to reduce development gamma, the linear velocity ratio of the sleeve must be lowered.
To obviate the adhesion of toner, it has been customary to incline the main pole angle of the magnet roller to the upstream side in the direction of rotation of the developer carrier. The main pole angle refers to an angle between a line connecting the axis of the developer carrier and the peak of the magnetic force of the main pole, which is normal to the developer carrier, and a line connecting the axis of the developer carrier and that of the image carrier. If the main pole angle is around zero degree, then the developer conveying speed at the nip is lowered and causes the pressure of the developer acting on the image carrier to increase. This makes it difficult to increase the linear velocity ratio of the sleeve and is apt to bring about adhesion. In this manner, none of the conventional schemes can obviate the adhesion of toner although improving tonality by reducing the rotation speed of the developer carrier.
How the toner adheres will be described hereinafter. Adhesion is apt to occur when the regular gap width for development decreases due to the oscillation of the image carrier and that of the developer carrier or when the developer is scooped up to the developer carrier in more than a regular amount. Assume that the amount of developer being scooped up to the developer carrier locally increases. Then, when such an amount of developer is conveyed through the nip while being pressed, a stress ascribable to the narrow gap causes the toner to dynamically soften or to thermally soften and melt or causes wax contained in the toner to dynamically ooze out. As the other toner or carrier begins to gather and core together with the softened and melted toner, such a portion forms a core with the result that the adhered portion grows little by little. The adhesion sometimes begins at substantially a single position and sequentially grows in the circumferential direction and sometimes grows at a plurality of positions.
The adhesion of the toner occurs even when its fluidity is lowered, e.g., when the lower content of the developer in the developing device increases or when the amount of stray toner whose amount of charge has been reduced due to the aging of the developer increases. Further, the adhesion of the toner occurs when the surface roughness and therefore conveying ability of the sleeve is reduced due to aging, when the sleeve temperature rises due to a hot environment or continuous copying, or when the sleeve is noticeably contaminated.
It is therefore an object of the present invention to provide an image forming apparatus capable of freeing the toner of a developer from adhesion.
In accordance with the present invention, in an image forming apparatus for scooping up a developer to a developer carrier, causing the developer to form a magnet brush on the developer carrier, and causing the magnet brush to contact an image carrier to thereby develop a latent image formed on the image carrier, the developer carrier includes a nonmagnetic sleeve and a stationary magnet roller disposed in the sleeve. The magnet roller has magnetic poles for conveying the developer and a main pole for causing the developer to rise from the sleeve. The main pole has a flux density of about 40% in the direction normal to the sleeve. The axis A of the developer carrier, the axis B of the image carrier and the peak C of the magnetic force of the main pole in the above direction are positioned such that an angle between a line AB and a line AC is substantially zero degree.
Also, in accordance with the present invention, in an image forming apparatus of the type described, the developer carrier includes a nonmagnetic sleeve and a stationary magnet roller disposed in the sleeve. The magnet roller has magnetic poles for conveying the developer and a main pole for causing the developer to rise from the sleeve. The main pole has a half-width of 22xc2x0 or less. The axis A of the developer carrier, the axis B of the image carrier and the peak C of the magnetic force of the main pole in the above direction are positioned such the an angle between a line AB and a line AC is substantially zero degree.
Further, in accordance with the present invention, in an image forming apparatus of the type described, the developer carrier includes a nonmagnetic sleeve and a stationary magnet roller disposed in the sleeve. The magnet roller has magnetic poles for conveying the developer and a main pole for causing the developer to rise from the sleeve. The magnet roller further includes an auxiliary magnet for helping the main pole form a magnetic force. The axis A of the developer carrier, the axis B of the image carrier and the peak C of the magnetic force of the main pole in the above direction are positioned such that an angle between a line AB and a line AC is substantially zero degree.